In many lighting systems, an alternating current (AC) ballast is used to deliver power from an AC power source, e.g. AC mains power (also referred to as line power or wall power or utility power) from the AC power grid, to one or more lighting devices (e.g., fluorescent lamps). In some systems, a so-called emergency lighting ballast device may be employed to supply power to the one or more lighting devices (e.g., lamps) power from the AC power source is lost.
FIG. 1 shows a functional block diagram of one example embodiment of a conventional lighting arrangement 10 including an AC mains ballast 15, a lamp 55, and an emergency lighting ballast device 101. FIG. 1 also shows a functional block diagram of one example of emergency lighting ballast device 101. As shown in FIG. 1, emergency lighting ballast device 101 includes an AC power outage detector 130, a back-up DC supply voltage (e.g., a battery 140), an inverter 150, a ballast 160, and relays 170 and 180. It should be understood that FIG. 1 shows a functional block diagram of only one exemplary embodiment of emergency lighting ballast device 101, and other configurations are possible. It should also be understood that emergency lighting ballast device 101 may include a number of additional components and/or functional blocks that are not illustrated in FIG. 1.
During a normal operating mode when AC mains power is available, AC power outage detector 130 detects the presence of an output voltage on AC ballast output lines 17 and controls relay 180 to connect the output voltage from AC mains ballast 15 across the terminals of the lamp 55 to supply power to lamp 55. Also during the normal operating mode, AC power outage detector 130 also controls relay 170 to disconnect the output of battery 140 from inverter 150, thereby conserving the charged state of battery 140.
In an “emergency” mode when AC mains power is lost, AC power outage detector 130 detects the absence of an output voltage on AC ballast output lines 17 and controls relay 170 to connect the output of battery 140 to inverter 150, thereby generating an emergency supply at the output of ballast 160. Also during the emergency mode, controls relay 180 to connect the emergency voltage output by ballast 160 across the terminals of the lamp 55 to supply power to lamp 55.
In some embodiments, in addition to or in place of detecting the presence or absence of an output voltage on AC ballast output lines 17, AC power outage detector 130 may detect the presence or absence of AC mains voltage directly, for example through the optional input 19 shown in FIG. 1. Due to the configuration shown in FIG. 1, an emergency lighting ballast device may be retrofitted into a lighting fixture that already includes an AC mains ballast and a lamp.
When designing the emergency lighting ballast device, a number of operating parameters are important, including the output power and output voltage supplied by the ballast. However, lighting systems may have a variety of different configurations and employ a variety of different lighting fixtures and lamps. The power requirements and/or start-up voltage requirements of one or more types of lamps employed in one lighting system may vary substantially from the requirements of one or more different types of lamps either within the same lighting system, or for a different lighting system. If an emergency lighting ballast device is employed that lacks an ability to be tailored to the lamp load to which it will be connected, lamp light output varies widely over the range of lamps operated. In this case, high power lamps would set the maximum load which the circuitry and the battery of the emergency lighting ballast device are capable of operating. This circuit and battery configuration means “smaller” lower power lamps are driven with less power, which means less light from the lamp. This is not ideal as the battery and circuit are capable of providing more power to these smaller lamps.
So it is desirable for an installer of an emergency lighting ballast device to be able to select the output power of the device to optimize the power delivered to a particular lamp. Furthermore, depending on the configuration, it may be desirable to provide the installer with an option for configuring an emergency lighting ballast device to drive either one lamp or two lamps (or more than two lamps, in some cases, if possible).
Thus, there is a need in the art for an emergency lighting ballast device which can be readily and flexibly configured to drive a variety of lamps and combinations of lamps.